Two reports originating from a research team headed by H. Seto describe a novel apoptosis-inducing macrolide from Nocardiopsis sp., designated apoptolidin. See, Kim, J., et al., J. Antibiot. (1997) 50:628-630; Hayakawa, Y., et al., J. Am. Chem. Soc. (1998) 120:3524-3525. These workers demonstrated that cancer cells could be selectively sensitized to apoptosis by this natural product. A truncated form of apoptolidin which lacks two of the natively occurring glycosylation residues is also described. The truncated form was prepared in 12% yield by treating native apoptolidin with acid in methanol. While only 1% of the cytotoxic activity of the apoptolidin itself was demonstrated in an assay involving FACS staining for Annexin V and by an MTT assay, at least with respect to the cell lines MCF-7, and the murine B lymphoma cell line LYas, the truncated form was more stable.
The Seto group demonstrated that apoptolidin induces apoptosis in cells expressing the E1A oncogene , but not in non-tumor cells or in cells expressing the ras or src oncogene.
It has now been found that the cellular target of apoptolidin, as well as that of other antifungals such as oligomycin, is mitochondrial ATP synthase. This enzyme is embedded in the mitochondrial membrane and the catalytic activity resides in a portion of the protein which extends inside the mitochondrion. It is responsible for phosphorylation of ADP in the oxidative phosphorylation reactions associated with aerobic metabolism.
Identification of this enzyme as a target provides a means to screen compounds for antitumor activity and elucidation of the mechanism by which apoptolidin acts to effect apoptosis provides a further rationale for the design of treatments for enhancing the effect of compounds which exert their antitumor effects in a manner analogous to that of apoptolidin.